We have developed methods of purifying leukocyte pyrogen from rabbit peritoneal exudate cells, and have raised an antibody to it in goats. We have also prepared leukocyte pyrogen labelled with 125I using the succinimide ester technique. We now wish to develop a radio immunoassay which can measure leukocyte pyrogen in the supernates of cells incubated in vitro, and in body fluids. We have also developed methods of partially purifying the pyrogen from rabbit macrophages. This pyrogen, and the Pyrogen from neutrophils, can be obtained free from all detectable endotoxin. We wish to see whether neutrophil and macrophage pyrogens have other biological actions besides causing fever. We also determine whether macrophage pyrogen is synthesized de novo from amino acid precursors.